The invention relates generally to the art of electric motors and motor drives and more particularly to an integrated DC link choke and methodology for suppressing common-mode voltage and smoothing DC current in a motor drive.
In the field of electric motors and motor drives, a motor is connected to a motor drive, which provides electrical power to the motor in a controlled fashion, wherein the motor drive may take a variety of forms, such as a current source inverter (CSI), voltage source inverter (VSI) or the like. Such motor drives are commonly employed to provide electrical power to motors. The motor drive may be employed in order to provide speed control, torque control, and/or to control other motor performance characteristics. For AC motors, electrical power is converted in the motor drive from supply power, typically AC power from a utility or other source, into DC. The DC power is then converted, for example using an inverter stage, into AC power at a controlled frequency and amplitude, which is provided to the motor windings.
The AC to DC converter stage in the motor drive, and/or the subsequent inverter stage, often comprise solid state semiconductor-based switching elements, such as silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate commutated thyristors (GCTs), insulated gate bipolar transistors (IGBTs), or other switching devices. As the switching elements are activated in the AC to DC converter stage or the inverter stage, common-mode voltages are produced. This common-mode voltage appears in the output phases of the motor drive, and hence, on the motor windings. Where a neutral associated with the input power source is grounded, this common-mode voltage appears between the motor windings and ground, and may reach levels beyond the motor winding insulation ratings. Consequently, such common-mode voltages may damage the motor or decrease the life expectancy thereof.
Conventional approaches to addressing this motor drive common-mode voltage problem have included designing the motor winding insulation to higher voltage ratings. However, this approach provides no solution for existing motors, which do not include insulation capable of withstanding the common-mode voltage levels. Another approach has been to provide an isolation transformer between the power source and the motor drive AC power input terminals. However, such an isolation transformer must be rated to handle the common-mode voltage levels. In addition, isolation transformers add cost and occupy space in the motor drive system. Thus, there is a need for apparatus and techniques to reduce or minimize the common-mode voltages in a motor drive, which do not require isolation transformers, and which operate in association with existing motors.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The invention provides a motor drive and a choke therefor, wherein the choke comprises a magnetic core with an inner leg and two outer legs, and four coils, which may be connected in DC current paths in the motor drive. First and second differential coils are wound around first and second outer legs, respectively; and first and second common-mode coils are wound around the inner leg of the choke. The first and second differential coils smooth the DC power on a DC bus in the motor drive, and the first and second common-mode coils suppress common-mode voltages in the motor drive. The invention thus provides an integrated DC link choke, which may be employed to provide both DC current smoothing and common-mode voltage suppression in a motor drive. In addition, the invention provides for a solution to the above motor drive common-mode voltage problems, whereby motors need not include extra insulation beyond the normal motor ratings, and whereby no external isolation transformers are required for a motor drive system.
According to one aspect of the invention, a choke provides a differential inductance and suppresses common-mode voltages in a motor drive having an AC to DC converter stage for converting AC power to DC power on a DC bus with positive and negative DC current paths, and an inverter stage for converting DC power from the DC bus to AC motor power in a controlled fashion. The choke comprises a magnetic core having an inner leg extending between an upper member and a lower member, and first and second outer legs extending from the lower member toward the upper member. A first air gap is provided between the first outer leg and the upper member and a second air gap is provided between the second outer leg and the upper member.
First and second differential coils are wound around the first and second outer legs, respectively, and may be connected in the positive and negative DC current paths of the motor drive, respectively, to smooth the DC power on the DC bus. The differential coils may be wound on the core to provide a differential flux in a differential flux path through the first and second outer legs, the first and second air gaps, and the upper and lower members of the core in response to differential current in the positive and negative DC current paths of the motor drive, whereby a differential inductance is provided to smooth DC power on the motor drive DC bus.
In addition, first and second common-mode coils are wound around the inner leg of the choke, which may also be connected in the positive and negative DC current paths, respectively, to suppress common-mode voltages in the motor drive. The common-mode coils provide a first common-mode flux in a common-mode flux path through the inner leg, the first and second outer legs, the first and second air gaps, and the upper and lower members of the core in response to common-mode current in the positive and negative DC current paths. The common-mode coils thus provide a common-mode inductance to suppress common-mode voltages in the motor drive. In addition, the differential coils may be wound around the outer legs of the core in order to provide a second common-mode flux in the common-mode flux path in response to common-mode current in the positive and negative DC current paths. The common-mode coils and the differential coils may thus provide a common-mode inductance to suppress common-mode voltages in the motor drive. The common-mode inductance and the differential inductance may be adjusted independently of one another according to a particular motor drive design.
Another aspect of the invention provides a motor drive for providing AC power to a motor in a controlled fashion. The motor drive comprises an AC to DC converter stage for converting AC power to DC power on a DC bus with positive and negative DC current paths, and an inverter stage for converting DC power from the DC bus to AC motor power in a controlled fashion. The motor drive further comprises a choke for providing a differential inductance and suppressing common-mode voltages in the motor drive. The choke comprises a magnetic core having an inner leg extending between an upper member and a lower member, and first and second outer legs extending from the lower member toward the upper member and providing a first air gap between the first outer leg and the upper member and a second air gap between the second outer leg and the upper member. First and second differential coils are wound around the first and second outer legs, respectively, and first and second common-mode coils are wound around the inner leg. The first and second differential coils are connected in the positive and negative DC current paths, respectively, to smooth the DC power on the DC bus, and the first and second common-mode coils are connected in the positive and negative DC current paths, respectively, to suppress common-mode voltages in the motor drive.
Yet another aspect of the invention provides a methodology by which common-mode voltages may be suppressed in a motor drive. A magnetic core is provided having an inner leg and two outer legs. First and second differential coils are wound around the first and second outer legs, respectively, and first and second common-mode coils are wound around the inner leg. The first differential and common-mode coils are connected in a positive DC current path in the motor drive, and the second differential and common-mode coils are connected in a negative DC path, so as to suppress common-mode voltages in the motor drive.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.